Mechanical reliability can be generally divided into structural reliability and mechanism reliability. Structural reliability mainly considers the strength of the mechanical structure and the failure caused by fatigue, wear and tear, fracture, etc. due to the influence of load; institutional reliability mainly considers the failure caused by the kinematics of the mechanism in the action process instead of the failure caused by the strength problem.
Mechanical reliability design can be divided into qualitative reliability design and quantitative reliability design. The so-called qualitative reliability design is based on the analysis of the impact of failure modes and hazards, and the targeted application of successful design experience to make the designed product reliable. The so-called quantitative reliability design is to design a product that meets the specified reliability requirements by establishing the relationship of implicit limit state function or explicit limit state function on the basis of fully grasping the strength distribution and stress distribution of the designed parts and the randomness of various design parameters.
The mechanical reliability design method is a commonly used method, and it is one of the most direct and effective methods to carry out mechanical reliability design at present. Both structural reliability design and institutional reliability design are commonly used in large numbers. Although quantitative reliability design can be used to design parts that meet the requirements in accordance with the reliability index, there is a lack of specific data on the strength distribution and load distribution of materials, and there are many factors to be considered, thus limiting its application, and it is generally used in the design of critical or important parts.
Mechanical reliability design Due to the differences in products and composition, the reliability design methods that can be used are
1. Failure prevention design
Mechanical products generally belong to series system. To improve the reliability of the machine, the first step should be to start from the strict selection and control of components. For example, give priority to standard parts and common parts; use reliable parts verified by the use of analysis; strictly according to the standard selection and control of outsourced parts; fully use the results of failure analysis, the use of mature experience or after the analysis of the test verification program.
2. Simplify the design
In the case of meeting the intended function, mechanical design should strive to simplify, the number of parts should be reduced as much as possible, the simpler the more reliable is a basic principle of reliability design, is the most effective way to reduce failure to improve reliability. But not because of the reduction of parts and other parts to perform extraordinary functions or work under high stress conditions. Otherwise, the simplified design will not achieve the purpose of improving reliability.
3. Derating design and safety margin design
Derating design is a design method to make the parts use stress lower than their rated stress. Derating design can be achieved by reducing the stress on the part or improve the strength of the part. Engineering experience has shown that most mechanical parts work under lower than rated stress conditions, their failure rate is lower and reliability is higher. In order to find the best derating value, a large number of experimental studies need to be done.
When the load stress of mechanical parts and the strength of the specific parts subjected to these stresses in a range of uncertain distribution, can be used to improve the average strength (such as through a large plus safety factor to achieve), reduce the average stress, reduce the stress variation (such as through the restrictions on the conditions of use to achieve) and reduce the strength variation (such as a reasonable choice of process methods, strict control of the entire processing, or through inspection or test to reject unqualified parts) and other methods to improve reliability. For the important parts involved in safety, you can also use the limit design method to ensure that it will not fail in the worst limit state.
4. Residual design
Residual design is the design of the structure, spare parts, etc. to complete the specified function, so that when local failure occurs, the whole machine or system will not lose the specified function. When the reliability requirement of a certain part is very high, but the current technology level is difficult to meet.
For example, the use of derating design, simplified design and other reliability design party sand soil, but also can not meet the reliability requirements, or improve the reliability of the parts of the improvement costs than repeated configuration is also high, the residual technology may become a better design method, such as the use of double pump or double engine configuration of the mechanical system, but it should be noted that the residual design often makes the whole machine volume, weight, cost are increased accordingly. Residual design improves the task reliability of the mechanical system, but the basic reliability is reduced accordingly, so the use of residual design should be cautious.
5. Environmental design
Environmental design is in the design of the product in the entire life cycle may encounter a variety of environmental influences, such as assembly, transportation shock, vibration impact, storage temperature, humidity, mold and other effects, the use of climate, dust vibration and other effects.
Therefore, it is necessary to carefully select design solutions and take the necessary protective measures to reduce or eliminate the impact of harmful environments. Specifically, it can be considered from three aspects: awareness of the environment, control of the environment and adaptation to the environment. Awareness of the environment refers to: should not only pay attention to the product’s working environment and maintenance environment, but also to understand the product’s installation, storage, transportation environment.
In the design and test process must simultaneously consider a single environment and a combination of environmental conditions; should not only be concerned about the natural environment in which the product is located, but also consider the environment induced by the use of the process. Control environment means: when the conditions allow, a good working environment condition should be created for the designed parts within a small range, or artificially change the environmental factors that are unfavorable to the reliability of the product. Adaptation to the environment refers to: in the inability to artificially control all environmental conditions, in the design scheme, material selection, surface treatment, coating protection and other measures to improve the ability of mechanical components themselves to withstand the environment.
6. Ergonomics design
The purpose of ergonomic design is to reduce the use of human error, play the respective characteristics of people and machines to improve the reliability of mechanical products. Of course, human error in addition to their own reasons, the console, control and manipulation environment, etc. also have a close relationship with the human error. Therefore, ergonomic design is to ensure the reliability of the system to communicate to the human dwelling.
For example, the indication system not only monitor rely on, but also the way the display, the configuration of the display, etc., so that people are easy to accept without error; second, the control and manipulation system is reliable, not only instruments and machinery have satisfactory accuracy, but also suitable for human use habits, easy to identify the operation, not easy to make mistakes, and safety-related, more should be anti-misoperation design; third, the design of the operating environment as suitable as possible for human work needs, to reduce Causes fatigue, interference with the operation of factors such as temperature, humidity, air pressure, light, color, noise, vibration, sand and dust, space, etc.
7. Robust design
The most representative method of robust design is the Taguchi method created by Dr. Genichi Taguchi, Japan, that is, the so-called design of a product should be completed by three designs of system design, parameter design and tolerance design, which is a kind of optimization design in the design process fully examining the internal and external interference that affects its reliability. This method has been developed by the U.S. Air Force in the RM2000 as an effective method to resist variation in design as well as to improve reliability.
8. Probabilistic design method
The probabilistic design method is based on the stress-strength interference theory, which treats stress and strength as random variables subject to a certain distribution.
9. Trade-off design
Trade-off design refers to a comprehensive trade-off between reliability, maintainability, safety, functional weight, volume, cost, etc., in order to find the best result.
10. Simulation method design
With the development of computer technology, the simulation method is becoming more and more perfect, and it can be used not only for the quantitative design of reliability of mechanical parts, but also for the quantitative design of reliability at the system level. Of course, the method of mechanical reliability design must not leave the traditional mechanical design and some other optimization design methods, such as mechanical computer-aided design, finite element analysis, etc.